chapter 3b chemical formulae
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ChemicalChemicalFormulaeFormulae
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Chemical formula is a representation
of a chemical substance.
NH3Each ammonia molecule consists of 1
nitrogen atom & 3 hydrogen atoms.
Number of
hydrogen atoms
Types of atoms
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Based on chemical formula, we
know
the composition of a substance
the elements that make up thesubstance.
the ratio ornumber of atoms of
each element in the substance.
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Compound Chemical
Formula
Elements
present
Ratio of numbers
of atom
Carbon
dioxide
CO2
Water H2O
Ammonia NH3
Sulphuric
acidH2SO4
Magnesium
hydroxide
Mg(OH)2
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Molecular FormulaMolecular Formula
Shows the actual number of
atoms of each element present in
one molecule of the compound.
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Empirical FormulaEmpirical Formula
Gives the simplest whole number
ratio of atoms of each elements
present in the compound.
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Example :
Hexene :
Molecular formula : C6H12.
Empirical formula : CH2.
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Example :
Dinitrogen tetraoxide :
Molecular formula : N2O4.
Empirical formula : NO2.
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Example :
Glucose :
Molecular formula : C6H12O6.
Empirical formula : ?
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Example :
Ethanoic acid :
Molecular formula : CH3COOH.
Empirical formula : ?
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Example :
Vitamin C :
Molecular formula : C6H8O6.
Empirical formula : ?
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Example :
Carbon dioxide :
Molecular formula : CO2.
Empirical formula : ?
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Empirical Formula Molecular Formula
Similarities
Both show the elements present in the
compound.
Both show the ratio ofatoms of each
elementin the compound.
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Empirical Formula Molecular Formula
Differences
Shows the simplest
ratio ofatoms of
each elements in
a compound.
Shows the actual
ratio ofatoms of
each elements in
a molecule.
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Determining Empirical FormulaeDetermining Empirical Formulae
By experiments.
Steps :
1. Find the mass of each element
2. Convert to moles of atoms
3. Find the simplest ratio of moles
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Example 1
1.08 g of aluminium combines chemically
with 0.96 g oxygen to form an oxide.
What is the empirical formula of theoxide ?
[ Relative atomic mass: O, 16 ; Al, 27 ]
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Example 2
1.69 g of iron combines chemically
with 0.72 g of oxygen.
Find the empirical formula of the oxide.[ Relative atomic mass: O, 16 ; Fe, 56 ]
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Example 3
0.91 g of aluminium burns in air to
form 1.7 g of aluminium oxide.
Find the empirical formula of theoxide.
[ Relative atomic mass: O, 16 ; Al, 27 ]
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Example 7
The decomposition of 7.36 g of acompound produces 6.93 g of oxygen.The rest of the mass is hydrogen.
If the relative molecular mass of thiscompound is 34.0 g, calculate itsmolecular formula.
[ Relative atomic mass : H, 1 ; O, 16 ]
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Determining empirical formula of
Magnesium oxide
A crucible & its lid are weighed.
A magnesium ribbon is cleanedwith sand paper remove the oxide layer.
The ribbon is coiled & is placed
in the crucible. The content isweighed.
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The crucible is heated
strongly without the lid.
When magnesium starts
to burn, the crucible is
covered with its lid.
Using a pair of tongs, the
lid is raised a little at
intervals allow oxygen
to enter the crucible &react with Mg.
The crucible is then quickly
covered with its lid to
prevent the white fumesfrom escaping. This would
affect the accuracy of the
mass obtained.
When burning is complete,
the lid is removed & the
crucible is heated strongly
for 1 to 2 min.
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The crucible is
allowed to cool toroom temperature
with its lid on.
The crucible & itscontent is weighed
again.
The process of
heating, cooling &weighing are
repeated until a
constant mass is
obtained ensurethat the reaction is
complete.
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Example 4
Phosphoric acid has the percentage
composition as follows :
H, 3.06% ; P, 31.63% ; O, 65.31%What is the empirical formula of the acid?
[Relative atomic mass: H, 1; O, 16; P, 31]
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Example 5
The percentage composition ofcalcium carbonate :
Ca, 40% ; C, 12% ; O, 48%
What is the empirical formula ofcalcium carbonate ?
[Relative atomic mass: Ca, 40 ; C, 12;
O, 16]
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Example 6
The empirical formula of ethene is(CH3)n. Its molecular mass is 30.
Calculate the formula of this
compound.
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Determining empirical formula of
copper (II) oxide
Zinc
Hydrochloric
acid
Combustion tube
(brown)
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Mass of combustion tube with porcelain
dish is weighed. A spatulaful of copper (II) oxide is added
on the porcelain dish. Weigh again.
Hydrogen gas is allowed to flow into thecombustion tube to remove air in the tube.
Function ofanhydrous calcium chloride is
to dry the hydrogen gas.
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Air in the combustion tube must be totally
removed before heating. A mixture ofhydrogen gas & oxygen gas will cause an
explosion when lighted.
To determine whether all the air has beenremoved, gas that comes out from small
hole is collected in a test tube. Gas is
tested with a lighted wooden splinter. Gas
burns without pop shows air has beentotally removed.
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Excess hydrogen gas flows out from small
hole is burnt to prevent air from enteringthe tube again.
Copper (II) oxide (black) is heated strongly
until it turns completely brown (copper). Flame is turn off but the flow of hydrogen
is continued until it is cool or the hot
copper produced will react with oxygen
from the air to form copper (II) oxide again.
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Mass of combustion tube with copper is
weighed again. The process ofheating, cooling &
weighing are repeated until a constant
mass is obtained. This is to ensure that all
copper (II) oxide has changed into copper.
This method applicable for oxides oflow
reactivity metals : tin (II) oxide, lead (II)
oxide.
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Mg + copper (II) oxide ?
Cannot. Both the reactants & products aresolids & thus the mass of copper & oxygen
cannot be determined.
Mg(s) + CuO(s) MgO(s) + Cu(s)
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Ionic Formulae
Made up ofpositively-charged ions (cations)& negatively-charged ions (anions).
Zinc bromide
Zn2+ Br
ZnBr2
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Cation Formula Anion Formula
Sodium ion Na+ Chloride ion Cl
Potassium ion K+ Bromide ion Br
Zinc ion Zn2+ Iodide ion I
Magnesium ion Mg2+ Oxide ion O2
Calcium ion Ca2+ Hydroxide ion OH
Iron (II) ion Fe2+ Sulphate ion SO42
Iron (III) ion Fe3+ Carbonate ion CO3
2
Copper (II) ion Cu2+ Nitrate ion NO3
Ammonium ion NH 4+ Phosphate ion PO4
3
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Construct the chemical formula :
Sodium chloride
Iron (II) iodide
Potassium sulphate
Ammonium hydroxide
Copper (II) phosphate
Zinc carbonate
Magnesium nitrate
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Name the ionic compounds :
AgCl
Ba(NO3)2
CaSO4
K2CO3
(NH4)3PO4
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Homework :
Text book
Activity 3.9 (P.46)
Activity 3.10 (P.47)
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